An idea I just had

Here's my idea:
A random GURPS one-shot.

Everyone is given two index cards. The person "running" the game writes out a number of index cards equal to the number of people there. All say 'player' except for one that says 'GM'. Anyone who is not interested in being GM is asked to raise their hand, and they are dealt player cards. The rest of the cards are dealt randomly.
Meanwhile, each person writes on one of their cards a short idea for a setting, and on the other card, a fairly major advantage or disadvantage. The organizer checks these for repeats, and then shuffles them and deals them.
First, each person turns over their role card. The person who got the 'GM' card turns over his setting card. All of the players turn over the card with the advantage/disadvantage. The players must create characters that incorporate that advantage/disadvantage, and the GM has until the players finish character creation to come up with a basic adventure.

This could be a challenging creative endeavour. Assumably, the players will share their results (as they should), so the GM will know if he has to create a setting that is "Pirates of the South China Sea in 1780" and still incorporate the player who got the "cyborg arm" card. I'd be very tempted to run this as a one-shot at some point. Especially with the random GM bit.
I'd also imagine that this might be fun enough to lead into a longer series of sessions, if not a full campaign.